Estes C6-7 Upper Stage Model Rocket Engine Pack - Includes 3 Engines with Starters & Plugs (1615)PLEASE NOTE: These are 18mm diameter engines. Please refer to the instructions that came with your rocket (or the rocket's product description) to determine engine compatibility. Estes C6 7 engine pack includes 3 engines, 4 starters, and 4 starter plugs The C6 7 is an upper stage engine designed for model rocket flight. This engine can also be used as a single stage engine in lightweight rockets. Recommended for use with the Phantom Blue, Starship